1. Field of the Invention
The present invention relates to a tube flaw detecting method by which a flawed portion in a heat exchanger tube constituting a boiler furnace wall and the like is detected by use of two probes.
2. Description of the Prior Art
With reference to FIG. 11, description will be made on a prior art tube flaw detecting method for detecting a flawed portion in a heat exchanger tube and the like.
In a thermal power generation boiler, a furnace wall is constructed such that furnace wall tubes 2, which are heat exchanger tubes, are arranged in close proximity to each other. A furnace outer side is supported by a reinforcing plate 1 of metal material, a heat insulating material 3 is applied thereto for preventing heat loss from the furnace wall and a further outer side thereof is covered by a casing 8.
In a recent thermal power generation boiler, in order to effect load adjustments corresponding to differences in the power demands between day time and night time, there are made severe demands of frequent repeat starts and stops. Due to thermal stresses caused thereby, thermal fatigue cracks occur at welded portions of the reinforcing plate 1 and the furnace wall tube 2 on the outer surface of the furnace wall tube 2. Also, corrosion fatigue cracks occur in the vicinity of the welded portions on the inner surface of the furnace wall tube 2. Hence it is necessary to detect these cracks with certainty at periodic inspections and to renew the furnace wall tube 2.
In detecting these cracks occurring in the furnace wall tube 2, because the furnace outer side is covered by the heat insulating material 3 and the casing 8, it is impossible to apply an ultrasonic probe directly to the furnace wall tube 2 in that state.
In the prior art, therefore, crack inspection is done such that, though not illustrated, the heat insulating material 3 and casing 8 are once removed for access to a cracked portion. Alternatively, as shown in FIG. 11, water 11 is filled in the furnace wall tube 2. An ultrasonic transmitter probe 6 is applied to a surface on the furnace inner side of the furnace wall tube 2 to transmit an ultrasonic wave to pass through the tube wall of the furnace wall tube 2 and the water 11 in the furnace wall tube 2. A reflected wave, which is reflected by a circumferential crack 9, is detected by the same transmitter probe 6, which also has a receiver function at the same time.
Out of the above-mentioned two typical ways in the prior art for detecting a flaw portion in a heat exchanger tube and the like, the first mentioned way requires work for removing the heat insulating material and the casing portion and restoring them after the predetermined inspection has been done, which results in a problem of a high cost for carrying out the inspection. Also, the second mentioned way requires work for filling water in the furnace wall tube for inspection, which results in a problem that other work, such as welding work etc., cannot be done due to the existence of water while the inspection is being carried out.